Electrophotographic imaging members, i.e., photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
A latent image is formed on the photoreceptor by first uniformly depositing electric charges over the surface of the photoconductive layer by one of any suitable means known in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the surface of the photoconductor not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming an electrostatic latent image.
The photo-induced discharge of the photoconductive layer requires that the layer photogenerate conductive charge and transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer structures in which photoconductors perform both functions. These layers are formed on an electrically conductive substrate and may include an optional charge blocking layer and an adhesive layer between the conductive substrate and the photoconductive layer or layers. Additionally, the substrate may comprise a non-conducting mechanical support with a conductive surface. Other layers for providing special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging, or subbing layers to provide chemical sealing and/or a smooth coating surface may also be employed.
One common type of photoreceptor is a multi-layered photoreceptor having a structure comprising an electrically conductive substrate, an undercoat layer formed on the substrate, a charge generating layer applied on the undercoat layer, and a charge transport layer formed on the charge generating layer. The phrases “charge blocking layer” and “blocking layer” are generally used interchangeably with the phrase “undercoat layer.” U.S. Pat. No. 5,314,776 to Nomura, Fukuda, Nagasaki, and Suda entitled “Multi-layered Photoreceptor for Electrophotography” describes a process for manufacturing a photoreceptor comprising a substrate which comprises an electroconductive support or a support having an electroconductive film formed thereon; an undercoat layer including a material selected from the group consisting of silicon dioxide and other silicon oxides formed on the substrate; a carrier generation layer formed on the undercoat layer; and a carrier transport layer formed on the charge generation layer.
U.S. Pat. No. 6,479,202 to Shida, Uchino, and Itami entitled “Electrophotographic Photoreceptor, Electrophotographic Image Forming Method, Electrophotograhic Image Forming Apparatus and Processing Cartridge” describes an electrophotographic photoreceptor having on a support a resin layer comprising a siloxane resin formed by hardening a compound represented by Formula 1, 2 or 3, or a hydrolyzed product which has a structural unit having a charge transportation ability, wherein a ratio M1/M2 of the sum of the amount of moles M1 of the compound represented by Formula 1 and that represented by Formula 2 to the amount in moles of the compound represented by Formula 3 is within the range of from 0.01 to 1;Si(OR1′)4  Formula 1R1Si(OR2′)3  Formula 2R1R2Si(OR3′)2  Formula 3wherein the formulas R1 and R2 each represent an alkyl group having one to ten carbon atoms, a phenyl group, an aryl group, a vinyl group, an amino group, a γ-glycidoxypropyl group, a γ-methacryloxypropyl group, or a CnF2n+1C2H4— group, R1′, R2′, and R3′ each representing an alkyl group and the groups represented by R1′, R2′, and R3′ may be the same or different from each other.
U.S. Pat. No. 6,361,913 to Pai and Yanus entitled “Long Life Photoreceptor” describes an electrophotographic imaging member comprising a substrate, a charge generating layer, a charge transport layer, and an overcoat layer comprising a hydroxytriphenyl methane having at least one hydroxy functional group and a polyamide film forming binder capable of forming hydrogen bonds with the hydroxy functional group of the hydroxy triphenyl methane molecule, the charge transport layer being substantially free of triphenyl methane molecules.
An undercoat layer may be provided to cover up substrate defects, to improve print quality (such as to reduce or eliminate imagewise constructive interference effects known as “plywood effect”), to ensure environmental insensitivity, and/or to enable good electrical properties, e.g., block holes, transport electrons, enable cyclic stability, provide low surface potential residue of photo-induced discharge (Vr) and dark decay (Vdd), and improve coating uniformity.
For electrophotographic imaging systems which utilize uniform negative polarity charging prior to imagewise exposure, it is important that the undercoat charge blocking layer bleeds off negative charge while preventing positive charge leakage. In this case, the undercoat layer which is thick enough to cover up the roughened surface of the substrate is desired. Further, undercoat layers that are too thin are more susceptible to the formation of pinholes which allow both negative and positive charges to leak through the charge blocking and result in print defects. Also, when charge blocking undercoat layers are too thin, small amounts of contaminants can adversely affect the performance of the charge blocking undercoat layer and cause print defects due to passage of both negative and positive charges through the layer. Defects in the hole blocking layer, which allow both negative and positive charges to leak through, lead to the development of charge deficient spots associated with copy print-out defects.
Generally, undercoat layer formulations can be classified as dispersed undercoat layer solutions or homogeneous undercoat layer solutions. Dispersed undercoat layers comprise non-soluble particles in binders and solvents. Homogenous undercoat layers comprise charge conductive species soluble in binders and solvents. A known method for preparing dispersed undercoat layer solutions comprises mixing metal oxides with polymeric binders in an organic solvent. The metal oxides may comprise, for example, titanium oxide, zinc oxide, zirconium oxide, tin oxide and aluminum oxide, among others. A wide variety of polymeric resin binders have been employed for this purpose, such as, for example, polyimides, polyamides, polyacrylates, vinyl polymers and other specialty materials. The dispersion procedure is very time-consuming. In order to achieve good electrical properties, the metal oxide particles in the solution must be nanometer grade in size. Problematically, in the standing dispersed solution, the metal oxide tends to precipitate, causing macro-phase separation which results in non-uniform coatings.
The process for preparing homogeneous undercoat layers is generally more convenient than that for preparing dispersed undercoat layers. Generally, the process for preparing homogeneous undercoat layers comprises mixing the forgoing materials in the suitable solvents and applying the mixture to an electrically conductive substrate using suitable coating methods as known in the art. As an example, a three-component undercoat layer is described in U.S. Pat. No. 5,789,127 to Yamaguchi and Sakaguchi entitled “Electrophotographic Photoreceptor” (Fuji-Xerox). The three-component undercoat layer described therein requires moisture during curing.
For most dispersed undercoat layer formulations, such as, for example, that described in U.S. Pat. No. 5,612,157 to Yuh and Chambers entitled “Charge Blocking Layer for Electrophotographic Imaging Member,” the range of suitable materials is somewhat limited. Many polymeric materials have the particle size, density, and dispersion stability in the proper range, but they have refractive index values that are too close to the binder resin used in the charge blocking layer. Light scattering particles having a refractive index similar to the binder refractive index may produce light scattering insufficient to eliminate the plywood effect in the resulting prints. Selecting inorganic particles such as metal oxides, which typically have a higher refractive index than polymeric materials, to be the light scattering particles is problematic because inorganic particles such as metal oxides generally have higher densities than polymeric materials and thus can create a particle settling problem that adversely affects the uniformity of the blocking layer and the quality of the resulting prints. Also, since the electrical properties tend to deteriorate when the undercoat layer is provided at a thickness of greater than about 6 micrometers, there is a thickness limitation of about 6 micrometers.
“Plywood effect” is a problem inherent in layered photoreceptors and so termed because when the spatial exposure variation in an image formed on a photoreceptor appears in the output print it looks like a pattern of light and dark interference fringes resembling the grains on a sheet of plywood. The issue of plywood effect has been addressed in the prior art by increasing the thickness of, and hence the absorption of light by, the charge generating layer. For most systems, this leads to unacceptable tradeoffs. For example, for a layered organic photoreceptor, an increase in dark decay characteristics and electrical instability may occur. U.S. Pat. No. 4,618,552 to Tanaka, Sumino, and Toma entitled “Light Receiving Member for Electrophotography Having Roughened Intermediate Layer” describes a method for compensating for plywood effect by using a photoconductive imaging member in which the ground plane, or an opaque conductive layer formed above or below the ground plane, is formed with a rough surface morphology to diffusively reflect the light.
Another method for compensating for plywood effect is described in U.S. Pat. No. 5,052,328 to Andrews and Simpson entitled “Photosensitive Imaging Member with a Low-Reflection Ground Plane.” U.S. Pat. No. 5,052,328 describes a ground plane of low reflection material so as to reduce the reflections therefrom. U.S. Pat. No. 5,089,908 to Jodoin, Loce, Lama, Rees, Ibrahim, and Appel entitled “Plywood Suppression in ROS Systems” describes a multiple diode laser array used in a raster output scanning (ROS) system modified to reduce the effects of undesirable spatial exposure variation at the surface of certain types of layered, semi-transparent photoreceptors. The spatial absorption variation is later manifested as a plywood pattern formed on output prints derived from the exposed photoreceptor. The laser array is modified to form a merged scanning beam at the photoreceptor surface of two or more diode outputs, each output operating at a different wavelength than the other. In one embodiment, a plurality of diodes, each at a different wavelength, are sequentially addressed, and an image of each diode is scanned across the photoreceptor which results in an exposure distribution that would be similar to that formed by an incoherent beam.
The disclosures of the foregoing are hereby incorporated by reference herein in their entireties.
There is still a need in the art for improved photoreceptors that overcome or alleviate the above-mentioned and other problems and for an improved method for preparing such photoreceptors.